Gas turbine engines for aircraft achieve thrust by discharging hot gases through the exhaust nozzle. Efficient operation for multi-mission application dictates the use of variable area convergent/divergent nozzles. Variable convergent/divergent configuration is achieved in axisymmetric nozzles by the use of a plurality of circumferentially arranged flaps. Overlapping seals are located between adjacent flaps. The hinge arrangement must be provided between each convergent flap or seal and each divergent flap or seal. Variations may be made during operation, of the throat and terminal discharge openings.
Specifically, such variable geometry exhaust ducts for aircraft gas turbine engines frequently use axisymmetric arrangement of adjacent flaps to define the periphery of the exhaust duct. Overlapping seal members operate to seal between the adjacent flaps. An intermediate transverse hinge in the flap and seal members between a convergent section and a divergent section provides the desired convergent/divergent arrangement. The duct may accordingly be configured to define a variable throat area and variable exit area which is necessary for optimum engine performance, particularly in high speed aircraft installations using after burning for thrust augmentation.
The convergent/divergent flaps guide the engine exhaust. It is usual to also supply exterior flaps on a one-to-one basis with the divergent flaps, located outboard of the nozzle surrounding the nozzle. This protects the internal mechanism of the nozzle and provides a smoother appearance. It also provides a proper aerodynamic exterior avoiding cracks or openings which could induce flow perturbations.
Maximum thrust and operating efficiency of a gas turbine engine is achieved when the engine exhaust passes through the exhaust nozzle which controls the expansion of the exhaust gases. Maximum operating efficiency generally requires that the nozzle be configured to exit the exhaust stream at substantially the same pressure as the surrounding ambient atmosphere.
With aircraft operating both at subsonic and supersonic speed the exhaust nozzle pressure ratio varies over a wide range. Variable throat convergent/divergent nozzles are used to achieve proper operation for the various operating conditions.
In the divergent section, gas turbine engine nozzles incorporate a plurality of circumferentially arranged divergent flaps. Each of the divergent flaps has a “gas side”, the surface of the divergent flap exposed to the exhaust gas of the engine, and an “air side”, the surface of the divergent flap opposite to the gas side. Divergent seals are located between, and overlap, adjacent divergent flaps to prevent the escape of the exhaust gas through the gaps between the divergent flaps.
Such gas turbine engine nozzles operate with gas at an extremely high temperature level such that cooling of the nozzle surfaces is required. A known method includes passing a forced flow of cooling air through a liner which is lining both the convergent flaps and the convergent seals. This cooling flow is then dumped at the throat adjacent to the surface of the divergent section to film cool that section. Unfortunately, the cooling flow does not generally cool the lateral edge regions of divergent seals in a conventional axisymmetric nozzle configuration.
Accordingly, it is an object of the present invention to provide a nozzle configuration that overcomes the above-mentioned drawbacks and disadvantages.